This invention relates generally to precision pumping apparatus and, more particularly to a pump controller for accurately controlling the amount of fluid dispensed from the precision pumping apparatus.
There are many applications where precise control over the amount and/or rate at which a fluid is dispensed by a pumping apparatus is necessary. In semiconductor processing, for example, it is important to control very precisely the amount and the rate at which photochemicals, such as photoresist, are applied to a semiconductor wafer being processed to manufacture semiconductor devices. The coatings applied to semiconductor wafers during processing typically require a flatness across the surface of the wafer that is measured in angstroms. Many semiconductor processes today have requirements on the order of 30 angstroms or less. The rate at which processing chemicals such as photoresists are applied to the wafer and spun out through centrifugal force to the edges of the wafer has to be controlled in order to ensure that the processing liquid is applied uniformnly. It is also critical to control the rate and volume at which photoresist chemicals are applied to the wafer in order to reduce unnecessary waste and consumption. Many of the photochemicals used in the semiconductor industry today are not only toxic, but they are very expensive, frequently costing as much as $1,000 per liter. Thus, because of the cost of the chemicals as well as the difficulties in handling toxic materials, it is necessary to ensure that enough of the photoresist is applied to the wafer to satisfy processing requirements while minimizing excessive consumption and waste.
Another important requirement for semiconductor processing is the ability to repeatedly dispense a precisely controlled amount of processing chemical each time since variations in the amount of chemicals can adversely impact consistency from wafer to wafer. In the past, because of the unrepeatability as well as the inability to precisely control the amount of chemical being dispensed, many pumps had to dispense 50% to 100% more liquid than needed in order to ensure a sufficient quantity for processing requirements. This has resulted in waste and increased processing costs.
Conventional pumping apparatus are able to accurately dispense precise amounts of typical fluids. However, these conventional pumping apparatus cannot accurately dispense low viscosity, low dispense rate fluids and the conventional pumping apparatus will either cause a double dispense or a stuttered dispense of the low viscosity fluid. In particular, at the beginning of the dispensing cycle prior to the controlled dispensing of any fluid, a small amount of the low viscosity fluid, e.g., several microliters, may be undesirable ejected onto the wafer's surface resulting in an imprecise amount of fluid being dispensed. The problems of double dispensing and stuttered dispensing of these low viscosity, low flow rate fluids are caused by a variety of factors which are present in a conventional pumping apparatus. For example, pressure may be built up in the dispensing chamber of the pumping apparatus due to the closing of a barrier valve prior to dispensing which may force some fluid into the dispensing chamber and increases the pressure in the dispensing chamber. The extra fluid and hence the extra pressure in the dispensing chamber may cause the small amount of fluid to be ejected onto the wafer's surface at the start of the dispensing cycle. In addition, the timing of the control valves operation and the dispense system dynamics, such as tubing length, tubing diameter and nozzle size, in a conventional pumping apparatus may also contribute to the problem of the double or stuttered dispense of low viscosity, low dispense rate fluids.
It is desirable to provide low volume, low rate chemical dispensing pumping apparatus capable of precise and repeatable control of the rate and volume of low viscosity chemicals dispensed by the pumping apparatus, and it is to these ends that the present invention is directed.